What happens to the working of petrol engine when it is emptied and filled with diesel or to a diesel engine emptied and filled with petrol? Will the engine be able to operate and, if not, why not?

  • $\begingroup$ There is a good youtube video (that I can't find quickly) where they put diesel into a gasoline lawn mower and it won't start. They then put about 50% diesel in the lawn mower and it smokes but it runs. $\endgroup$ – user1683793 Feb 28 at 20:04

Putting diesel fuel in a gasoline engine is just about impossible. Diesel fuel nozzles are larger than gasoline nozzles, and modern gas caps are too small for diesel nozzles to fit into. However, if you managed to get it in there, the diesel fuel is too heavy and evaporates too slowly for the spark plugs to ignite it effectively. One source says that it won't start at all, another source says that it probably won't start, but there's a chance it will, it'll just run horribly and end up as a smokey disaster.

The converse is possible, since the gasoline nozzle is the smaller of the two. In this case, as noted in the second article linked above, gasoline would risk damage to the emissions system, and it will definitely lead to damage of the engine. Diesel fuel lubricates the engine as it is consumed. Gasoline, being thinner (part of the reason it evaporates easier), will not provide that lubrication and you'll start to see friction damage on engine components. Because it's also designed to combust differently, you'll see messed up timing, which can cause further damage.

It's worth noting that, in the increase in popularity of diesel cars and the lack of mindfulness of drivers, Volkswagen has altered its diesel tank gates to prevent gasoline from being put into the tank, because of the damage it can cause.


To understand how these fuels behave in internal combustion (IC) engines, you need to first understand the properties of the fuels and how they differ.

Fuel Properties

Both petrol (gasoline) and diesel are lighter-than-water organic liquids, most commonly produced from petroleum. Diesel requires much less effort to produce—you can make it yourself from cooking oil with only a few readily-available chemicals.

Gasoline is lighter that diesel, with a density about three-quarters that of water, and highly volatile. That means it evaporates quickly at room temperature, which is why it has such a strong odor and is stored in closed containers such as these when not fueling a vehicle directly. Apart from the odor, gasoline vapors are also very potent carcinogens and precursors for the formation of photochemical ("brown") smog, which during one particularly bad incident mistaken for a chemical weapons attack.

Diesel, in comparison to gasoline, is not very volatile at all. This is partly because it's less refined; the closer the distillate is to crude oil, the more stable it tends to be. You can store it diesel an open container for short periods with no significant losses.

Flash Point

Closely related to the volatility of the liquid is its flash point, the temperature at which the liquid's vapor pressure is high enough for its vapors to ignite in the presence of an ignition source. Below this temperature, the equilibrium state will be such that while you can burn the vapor, e.g. with a lit match, the combustion reaction would be localized. Above the flash point, the flame would cause a chain reaction that spreads throughout (ignites) the volume of vaporized fuel.

As you would expect, gasoline's high volatility results in a much lower flash point than diesel. Wikipedia gives the values as −43 °C (gasoline) and 52 °C (diesel). For reference, that means that if you filled an indoor swimming pool with gasoline and let the room come to equilibrium, you'd have to cool that room down below −43 °C (approximately Antarctic temperatures) before a lit match would not cause the room to explode. And even at a temperature well below the flash point, that match would burn really fast.

On the other hand, fill the same swimming pool with diesel fuel, and you'd have to heat the room above 52 °C (approximately Death Valley temperatures) before a lit match would cause the room to explode.

This is different from the autoignition temperature, also given in the Wikipedia article, which is the point at which you no longer need to light the match. Kaboom! Although gasoline has the lower flash point, it's diesel that has the lower autoignition temperature (256 °C, compared to 280 °C, from the WP article above).

As we move on from talking fuels to talking engines, the #1 point to remember is that with an ignition source, gasoline ignites first; without an ignition source, diesel ignites first.

Engine Properties

"IC engine" is a broad category comprising many engine types and technologies. The main difference between conventional diesel and gas engines has to do with how the fuel is ignited (see #1 point above!): gasoline engines typically use spark ignition and diesel engines use compression ignition.


Before ignition, the fuel in the tank needs to be mixed with air. This can be done in a variety of ways and although the general concept is simple, the fuel properties have a great impact on how this mixture is achieved.

Typically, the fuel is atomized in the cylinder—sprayed through small nozzles called fuel injectors—producing many very small droplets of fuel suspended in air. (Older cars used a different technology to create the fuel-air mixture.) The more homogeneous the fuel-air mixture (or "charge"), the more efficiently and completely it will burn and the better the engine will operate overall.

Our goal is to achieve a good fuel-air mixture quickly, ignite the fuel, extract its energy (in the form of useful work) and move on to the next stroke. This is how we get power (work over time) out of the engine. Atomizing the liquid fuel both disperses it and increases its surface area, allowing it to evaporate more quickly. Gasoline's high volatility compared to diesel makes it much easier to completely vaporize, obtaining that homogeneous charge. This is not necessarily always a benefit, though—which leads us to consider the properties of the engines themselves.


Spark ignition is perhaps the easier method to understand; it's comparable to the lit match analogy from earlier. The difference is that after vaporizing the fuel and mixing it with the air, the ignition source is electric spark rather than an open flame. Because the spark controls the exact time of ignition, we can make sure it happens at the best point during the piston's stroke to transfer power to the drive train. If ignition occurs at the wrong time, it could drive the piston in the wrong direction or burn the fuel incompletely. This is called knocking, and it can cause permanent damage to the engine.

Many additives have been used over the course of the past century to prevent gasoline from igniting without a spark, including lead and MTBE. Other formulations are designed to be used in high-performance gasoline engines (which have higher compression ratios) or to improve emission characteristics.

Compression Ignition

In a diesel engine, there is no spark; instead, the air in the cylinder is compressed rapidly, heating it up past the auto-ignition point of the fuel. Once there's enough thermal energy available to make the combustion reaction spontaneous, kaboom! You get your ignition. But it does take a lot of compression to reach that point—much more than is required for efficient operation of a spark-ignition engine. This is measured in terms of a ratio of the volume of the closed cylinder at the beginning and end of a stroke, also known as the compression ratio of the engine.

Without going into all the details of engine cycles, here's an animation that shows how the volume of the cylinder changes as the piston moves up and down:

4StrokeEngine Ortho 3D Small (CC BY-SA 3.0 or GFDL), by Zephyris (Own work), from Wikimedia Commons

A typical gasoline-powered spark-ignition engine has a compression ratio of about 10:1, meaning the volume of the cylinder at the start of the compression stroke is 10 times the volume of the cylinder at the end of the stroke. Diesel-powered compression-ignition engines have higher compression ratios, typically around 17:1 but may be higher. Both gasoline and diesel compression ratios vary from engine to engine; the thing to remember is that diesel engines have significantly higher compression ratios than their equivalent gasoline counterparts.

Using the Wrong Fuel

So, what happens when you use petrol (gasoline) in a diesel engine, or vice versa? A lot of things—but generally, what people care about are:

1. Will the engine run?

Diesel in a Gasoline Engine

As Trevor notes, there are safety measures in place to prevent filling a gasoline vehicle with diesel fuel. Diesel is also thicker than gasoline, which can mean clogged fuel lines and injectors, but let's assume that the diesel actually makes it into the engine.

Remember the #1 point from our discussion of fuels? When dealing with spark ignition, diesel is harder to ignite than gasoline. It has a much higher flash point, so we need to raise its temperature quite a bit before the engine has any chance of running. The compression ratio of the spark-ignition engine is lower, which means the fuel doesn't get as hot. As a consequence, it's very likely that the engine won't run at all.

If the engine has a high enough compression ratio, and/or if it's a hot enough day—remember, the flash point of diesel is somewhere around the high end of Death Valley temperatures—then the engine will run, but not very well. It also may not run for very long; you may achieve ignition in some cylinders but not others, in some strokes but not others, and eventually either the fuel itself or its partially-burned leftovers are going to clog up some part of the system. Even when you do get ignition, you'll get less power because the RPMs are higher (diesel burns more slowly than gasoline) and the temperature is lower.

Gasoline in a Diesel Engine

This is in many ways a more interesting case—it's easier to fit a gasoline nozzle into a diesel vehicle at the pump and the thinner gasoline may not have as much trouble moving through a fuel system designed for thicker diesel, so we can be confident that the fuel will reach the engine.

Now we're in the opposite situation from before. We have a fuel that not only is much easier to ignite via spark than compression (remember, its autoignition temperature is higher than diesel's), it's also formulated with additives to prevent it from igniting without a spark so that it won't damage the engines it's designed to work in. And so again, there is the possibility that the engine may not run at all.

But that doesn't necessarily mean no ignition! I'm going to quote The Straight Dope for this part of the explanation:

Since gasoline is designed to be resistant to self-ignition, gasoline in a diesel engine either won't ignite or will ignite at the wrong time. Some diesel engines run leaner than gasoline engines (meaning that the air-fuel mix has a higher proportion of air than a gasoline engine). That increases the chances that the gasoline won't ignite and that unburnt fuel will be sent into the hot exhaust system--where, ironically, it could ignite, leading to possible exhaust damage.

This leads us to:

2. Will the engine be damaged?

Diesel engines are often perceived as "tougher"—they're built to withstand the much higher pressures that come with their high compression ratios and, at least in the US, they're generally marketed in larger vehicles and trucks. They deliver more torque, they move heavy loads, they're used in generators and industrial equipment; so it's ironic that putting gasoline in a diesel engine is much more likely to cause major damage to the vehicle than the converse.

In addition to the risk of gasoline vapors passing through the engine to explode in other systems, if the engine does manage to ignite the gasoline in the cylinder, Trevor pointed out that the thinner gasoline will provide less lubrication to the engine; even at diesel RPMs, it won't take too long for the engine to tear itself apart without proper lubrication. The Straight Dope article also mentions loss of lubrication potentially damaging the fuel pump.

Bottom Line

Either way, it's possible to ignite the fuel and drive the pistons, meaning the engine could run—poorly. But it's much better for the vehicle if the engine does not run—especially for the diesel vehicle, which is more likely to be damaged by the gasoline (and could theoretically explode!). We are not here to give out auto maintenance advice, but if you understand the problem from an engineering perspective, it should be clear that the best case scenario here is that you discover the problem before starting up the vehicle, so that the fuel tank can be drained and refilled with the correct fuel type.

And remember, kids: Don't try this at home!

  • $\begingroup$ Great answer, and link of exploding balloon. Perhaps if you do try it at home, don't try it in your living room. Hahahaha! $\endgroup$ – McGafter Nov 16 '15 at 11:03
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    $\begingroup$ Basically, you can sum the damage to SI engines from CI fuels up as CI fuels being effectively 0 AKI -- some CI fuels (Jet-A, most notoriously) will happily make it through a SI engine's fuel system to the engine, with rather...bad results. $\endgroup$ – ThreePhaseEel Dec 10 '15 at 2:47

IC engines work in cycles, in a four-stroke IC engine there are four cycles

  1. Intake stroke
  2. Compression stroke
  3. Combustion stroke
  4. Exhaust stroke

During intake stroke the air-fuel mixture is sent into the engine cylinder, later in the compression stroke, it is compressed and then in the ignition stroke fuel gets ignited to take out power. In the exhaust stroke, the residue is sent out of the chamber.

There are two types of IC Engines based on their method of fuel ignition

  1. SI (spark ignition: typical gasoline or petrol engines)
  2. CI (Compression Ignition: typical diesel engines)

The way they are named like that is because in SI engines the fuel is ignited using a spark plug and in CI engines fuel is compressed to such high pressure that it reaches a temperature near to its autoignition temperature.

*This auto-ignition temperature is an important thing to notice

The autoignition temperature or kindling point of a substance is the lowest temperature at which it spontaneously ignites in normal atmosphere without an external source of ignition, such as a flame or spark.

table with different fuels' auto-ignition temperatures

As shown in the figure, it is clear that diesel has a lower auto-ignition temperature than gasoline. For diesel, it is 210°C; for gasoline, it is 280°C.

In CI engines the air is taken during the intake stroke and it is compressed to a high pressure where the air will reach a temperature of more than 210°C, then diesel is injected using a fuel injector. As soon as the diesel comes in contact with the air it gets ignited.

Now when you put gasoline inside the diesel engine, the air during compression still will be at a temperature ranging 210°C - 220°C and the Gasoline entered has an auto-ignition temperature of 280°C. This makes fuel impossible to ignite, hence the engine will not start. If it starts it will stop within seconds due to the accumulation of fuel in the cylinder.

Whereas SI Engines use a different method to ignite the fuel, first the fuel is mixed with air in the carburetor. This mixing can take place only when the fuel is in vapour state. For that, the flash point of the fuel must be very low. (flash point is the lowest temperature at which a liquid can form an ignitable mixture in air near the surface of the liquid. The lower the flash point, the easier it is to ignite the material.)

Now coming to flash point of gasoline it is -44°C at normal atmosphere temperatures it can easily form vapours. But diesel has a flash point of 55°C. When you put diesel into the petrol engine (SI) it will not form an ignitable mixture with air, even when a spark is generated inside the cylinder the diesel inside will be in liquid form which is not the ideal condition for flame generation. Thus the engine will not get started.

That is the reason why the vehicles are not compatible with other fuels than they are designed for.

*NOTE:- This is most raw explanation for this question, more in-depth explanations may be there please refer to them as well.

Fun To Know:- Due to higher compression ratios diesel engines are usually used for high torque applications like load carrying and power generation. Petrol or gasoline engines are used in commercial cars and bikes due to their light weight and faster response.


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